A Theoretical Study of the Recently Suggested MnVII Mechanism for O-O Bond Formation in Photosystem II.

Xi-Chen Li,Jing Li,Per E M Siegbahn

doi:10.1021/acs.jpca.0c05135

Abstract

The mechanism for water oxidation in photosystem II has been a major topic for several decades. The active catalyst has four manganese atoms connected by bridging oxo bonds, in a complex termed the oxygen-evolving complex (OEC), which also includes a calcium atom. The O–O bond of oxygen is formed after absorption of four photons in a state of the OEC termed S4. There has been essential consensus that in the S4 state, all manganese atoms are in the Mn(IV) oxidation state. However, recently there has been a suggestion that one of the atoms is in the Mn(VII) state. In the present computational study, the feasibility of that proposal has been investigated. It is here shown that the mechanism involving Mn(VII) has a much higher barrier for forming O2 than the previous proposal with four Mn(IV) atoms.

Highlights

Water oxidation using sunlight is one of the most important processes in nature.[1,2] Decades of research have been dedicated to find out how this remarkable reaction occurs.Large theoretical and experimental efforts have been able to reach a high level of consensus on even the finest details of the mechanism
The structures of the four observable intermediates been determined to high pinrecthiseiopnr.5o−c1e0ssT, hSe0 to S3, have most recent experimental determinations of the structures have been done using the X-ray free electron laser (X-FEL) technique
One of the leading suggestions was that the O−O bond was formed by a nucleophilic water attack on a terminal oxo group bound to the oxygen-evolving complex (OEC).[11−13] During more recent years, that mechanism has been criticized, both by theory and by experiments

Summary

■ INTRODUCTION

Water oxidation using sunlight is one of the most important processes in nature.[1,2] Decades of research have been dedicated to find out how this remarkable reaction occurs. One of the leading suggestions was that the O−O bond was formed by a nucleophilic water attack on a terminal oxo group bound to the OEC.[11−13] During more recent years, that mechanism has been criticized, both by theory and by experiments. The leading mechanism is that the O−O bond is formed between a radical oxyl group and a bridging oxo ligand.[14−19] It can be added that there exists another suggestion for O−O bond formation with similarities to the present suggestion.[48]. A new mechanism was recently suggested by Sun and Zhang[20] In that mechanism, the key step is the formation of a Mn(VII) di-oxo site on the dangling manganese in the S4 state. In previous studies of photosystem II (PSII),[16,26] it has been found that the fraction of 15% is the Received: June 6, 2020 Revised: August 30, 2020 Published: September 2, 2020

■ RESULTS

■ SUMMARY

■ ACKNOWLEDGMENTS

■ REFERENCES